In certain applications it is desirable to determine the location of an emitter (e.g., cell phone) of a radio transmission signal. Precise location information in a cellular telephone network is desirable for various reasons. Location information can be employed to aid hand-off, increase signal sensitivity, reduce co-channel interference and increase cell capacity. Additionally, location information is utilized in 911 dispatching, tracking unauthorized cell phone usage, and tracking or locating commercial and government vehicles. The FCC has required that Phase I of a wireless emergency 911 system (E-911) provide a 911 agent (a Public Safety Answering Point) with caller number and cell site location, while phase II of the E-911 system requires that caller latitude and longitude with an error radius of not more than 125 meters be provided. These techniques are designed to locate a cell phone to a building, but cannot give the location of the cell phone within the building (e.g., within a room). Therefore, current E-911 system techniques are not precise enough to locate a cell phone in a search and rescue operation. Additionally, current techniques proposed to locate cell phones assume that a cell phone is in use and has an active link with a cell tower.
Another problem with locating cell phones with high precision is due to multipath. Multipath is caused by the reflection of signals from objects in the environment, such as buildings, hills and other structures. A signal transmitted from a cellular phone can result in many multipath signals arriving at a base station in addition to a direct path signal. Therefore, signals from a phone can appear to arrive from multiple directions and at different times from a main signal. Another source of impairment is additive noise or interference from other cellular phones and base stations. For example, many cellular systems transmit signals on the same frequencies and time slots. The Code Division Multiple Access (CDMA) standard employs the same set of frequencies at each cell, while other systems reuse each frequency within certain geographical constraints. Therefore, it is difficult to detect a weak signal, for example, in a pile of rubble, amongst the other interfering signals due to multipath and other cell phone signals within the same frequency range.
As the events of 11 Sep. 2001 have demonstrated, there exists today no practical method to locate quickly and accurately a cell phone or other radio transmitter in a pile of building rubble. Such quick and accurate location estimates could guide rescue efforts and thereby save lives. Various attempts to solve some of these problems using Global Position System (GPS) technology have been made, but fail for various reasons. For example, most of today's cell phones have no capability to receive GPS signals. Additionally, GPS signals are too weak to be received inside most buildings, and location estimates derived from GPS signals received inside buildings have greatly degraded accuracies due to multipath reflections. Even under the best conditions, GPS fundamentally does not provide the location accuracy required for rescue operations.